1. Field of the Invention
This invention relates to a process for purifying tryptophan.
2. Description of the Related Art
Tryptophan is conventionally obtained from a fermentation liquid or an enzyme reaction product by condensation or by neutralization after cooling and subsequent liquid-solid separation. However, by this conventional method, impurities in the fermentation liquid or in the enzyme reaction product are taken into the crystals of tryptophan, so that it is impossible to obtain tryptophan of high purity.
A method in which tryptophan is recrystallized in the presence of a lower alcohol or a ketone is disclosed in Japanese Laid Open Patent Application (Kokai) No. 39857/84. However, with this method, tryptophan having satisfactory high purity cannot be obtained.
Japanese Laid Open Patent Application No. 126070/86 discloses a method of purifying tryptophan, in which a non-polar porous resin is used. However, in this method, an expensive resin is required. Moreover, the obtained tryptophan solution is very dilute, so that a substantial amount of energy is required for its condensation. Thus, this method is not economical.
In general, to obtain tryptophan with high purity, condensation, recrystallization and solid-liquid separation are required after once dissolving tryptophan so as to remove insoluble impurities. However, since the solubility of tryptophan in water or alcohol is small, a large amount of solvent is required for dissolving tryptophan, so that a large amount of energy is required for the condensation. Moreover, during the condensation, tryptophan is decomposed, so that the purity and yield of tryptophan are reduced.